Field of the Invention
The invention concerns nonlinear optically active copolymers and also electro-optical and photonic components including such copolymers.
Polymers with nonlinear optical properties (NLO polymers) are known to the art. Such materials are employed, for example, as electro-optical switches and are used in applications in areas of data processing and integrated optics, such as optical chip-to-chip connections, waveguiding in electro-optical layers, the Mach Zehnder interferometer and optical signal processing in sensorics.
An overview of current problems associated with the development of materials with pronounced nonlinear optical properties has appeared in "Angewandte Chemie", Vol. 107 (1995), pp. 167 to 187. In addition to the requirements which must be met by the chromophores with nonlinear optical properties, problems are also pointed out which arise in the development of polymeric matrices for embedding the chromophores and the stable alignment of their orientation.
In order for polymers provided with covalently bound or dissolved nonlinear optical chromophores to become nonlinearly optically active and to possess a high 2nd order susceptibility, the chromophores must be oriented in the electric field (cf. J. D. Swalen et al. in J. Messier, F. Kajzar, P. Prasad "Organic Molecules for Nonlinear Optics and Photonics", Kluwer Academic Publishers, Dordrecht, 1991, pp. 433 to 445). Orientation normally proceeds in the region of the glass transition temperature, where the mobility of the polymer chain segments enables orientation of the nonlinear optical chromophores to take place. The orientation achieved in the field is then frozen by cooling or better by cross-linking the polymer.
The 2nd order susceptibility .chi..sup.(2) thereby attainable is proportional to the space density of the hyperpolarizability .beta., to the ground state dipole moment .mu..sub.0 of the chromophores, to the electric polarization field and to parameters which describe the orientation distribution after the polarization process (cf. K. D. Singer et al. in P. N. Prasad, D. R. Ulrich "Nonlinear Optical and Electroactive Polymers", Plenum Press, New York, 1988, pp. 189 to 204).
Compounds with both a high dipole moment and high .beta. values are of great interest. For this reason investigations have been made especially of chromophores which consist of conjugated .pi.-electron systems which carry an electron donor at one end of the molecule and an electron acceptor at the other and which are covalently bound to a polymer, for example to polymethyl methacrylate (U.S. Pat. No. 4,915,491), polyurethane (European Patent Application EP 0 350 112) or polysiloxane (U.S. Pat. No. 4,796,976).
Known polymeric materials with nonlinear optical properties have the disadvantage, however, that relaxation of the oriented chromophore constituents takes place with the resulting loss of the nonlinear optical activity. This relaxation effect currently prevents the preparation of technically useful electro-optical components possessing long-term stability.
A further disadvantage of known polymeric materials with nonlinear optical properties is that it is not possible to modify the value of the NLO coefficient and other important parameters such as refractive index and glass transition temperature. As a result of their chemical structure, the chromophoric systems previously described also possess insufficient thermal stability to withstand without damage the thermal stresses arising during the fabrication and/or use of the electro-optical and photonic components. Thus a significant decrease in the measured electro-optical coefficients already takes place at a temperature of 85.degree. C. as a result of the relaxation process of the chromophores in the polymer matrix. Stability of the optical coefficients at temperatures above 100.degree. C. is actually desirable and for this reason a significantly higher glass transition temperature is needed for the polymeric material.